In connection with the production of such coated papers or LWC papers, the properties of the pulps so utilized must meet certain very strict requirements. The reason for this is primarily because the paper must have high density, low roughness, low porosity, as well as high strength. Furthermore, a uniform surface structure of these papers is of particular importance.
Paper of this type normally includes both chemical and mechanical pulps. The traditional mechanical pulp component used in this paper has been a groundwood pulp. However, as an alternative, thermomechanical pulp (TMP) has been used in recent years, but with limited success. Experience has thus proven that the employment of such TMP as a mechanical pulp component, even at low freeness values, results in a certain unevenness in the surface structure of the paper. This, in turn, gives rise to poor coating and, thus, to unacceptable printability. It has only been possible until now to avoid these problems by the paper manufacturer taking special steps to either modify or eliminate entirely the negative effects of that fraction in the thermomechanical pulp which constitutes long fibers. This long fiber fraction thus includes long, rigid and unworked fibers, which have a negative effect on the surface structure of the papers produced therefrom.
Most recently, chemically modified TMP (CTMP) has arisen as an attractive alternative to TMP as a component of the mechanical pulp in such LWC-papers and papers of similar quality. It has thus been found that CTMP improves the strength and binding properties thereof, and permits a reduction in the aforenoted problems with long fiber fractions. This is due to the fact that the mild chemical pretreatment used therein substantially improves both the defibering capability of the wood material and the tendency of the fibers in the long fiber fraction of the pulp to swell. These fractions, in turn, provide for a lower shives content and improved flexibility and ductility for the long fibers in CTMP, as compared to TMP.
Both the TMP and CTMP intended for use in LWC-papers and the like are generally manufactured by refining in one or more steps, and then screening and bleaching the refined material. The energy consumption in such a process is relatively high, and considerable equipment for dewatering and washing is required. Therefore, the search has continued for improved methods of producing such papers.